1. Field of the Invention
The present invention relates to a non-aqueous pigment ink, and relates particularly to a non-aqueous pigment ink that is suitable for use in an inkjet recording system.
2. Description of the Related Art
The coloring materials for inks used in inkjet recording systems can be broadly classified into materials that use pigments and materials that use dyes, and there is a growing tendency for the use of inks that use pigments as the coloring materials, as such inks exhibit the excellent levels of light resistance, weather resistance and water resistance that are required for high image quality printing.
In terms of the solvent, inks can be broadly classified into aqueous inks and non-aqueous inks. Non-aqueous inks that do not use water as the ink solvent, including solvent-based inks that use a volatile solvent as the main constituent and oil-based inks that use a non-volatile solvent as the main constituent, exhibit superior drying properties to aqueous inks, and also display excellent printability.
In non-aqueous inks, a pigment dispersant that dissolves in the solvent is generally used, but because this pigment dispersant improves the affinity between the solvent and the pigment, when the solvent penetrates into the recording paper, the pigment tends to be also drawn into the interior of the recording paper. As a result, the print density tends to fall, and show-through becomes more prevalent.
Accordingly, Patent Document 1 has proposed a non-aqueous pigment ink that uses non-aqueous resin dispersion particles (NAD particles) having a pigment dispersion capability as a dispersant. By using a pigment dispersant that does not dissolve in the solvent, this non-aqueous ink is able to provide improved print density for printed items on plain paper. However, further improvements are still required in terms of the pigment dispersion stability.
Furthermore, using a NAD as the pigment dispersant tends to cause an increase in the ink viscosity, and high-viscosity inks tend to suffer from problems of inferior storage stability and discharge stability when used in inkjet recording systems. Moreover, the amount of pigment that can be added to the ink tends to be limited, making it difficult to obtain a printed image of high density.    Patent Document 1: Japanese Patent Laid-Open No. 2007-197500